Metal and metal alloy powders have many important applications, especially in electronics and dental industries. Mixtures of palladium and silver are widely used in conductor compositions for hybrid integrated circuits. They are less expensive than gold compositions, are compatible with most dielectric and resistor systems, and are suitable for ultrasonic wire bonding. The addition of palladium to silver greatly enhances the compatibility of the circuit for soldering, raises the melting point of the silver for compatibility with the dielectric firing temperatures and reduces the problems of silver migration which can cause degradation of the dielectric properties and shorting.
Silver powder, palladium powder, mixtures of silver and palladium powder, and silver-palladium alloy powders are used in electrode materials for multilayer ceramic capacitors. The properties of the metallic components of thick film inks intended for the internal electrodes of multilayer ceramic capacitors are extremely important because compatibility is required between the metal powder and the organic medium of an ink and between the ink itself and the surrounding dielectric material. Metal particles that are uniformly sized, approximately 0.1-1.0 microns in diameter, pure, crystalline, and unagglomerated are required to maximize the desired qualities of a conductive thick film paste.
Printed circuit technology is requiring denser and more precise electronic circuits. To meet these requirements, the conductive lines have become more narrow in width with smaller distances between lines. This is especially true where multilayer ceramic capacitors are requiring thinner and narrower electrodes. The metal particles necessary to form dense, closely packed, narrow lines must be as close as possible to monosized, fully dense, smooth spheres. The conductive metal particles must have a small particle diameter, an even grain size and a uniform composition. In general, mixtures of silver and palladium powders are used to form the correct ratio silver-palladium powder. After the conductor lines are printed and fired, the silver and palladium particles are alloyed. As the printed lines get smaller, the requirements for homogeneity become much more important. To insure homogeneity of the alloy, it is preferred to start with a fully dense silver-palladium alloy powder at the desired ratio.
Many methods currently used to manufacture metal powders can be applied to the production of silver-palladium powders. For example chemical reduction methods, physical processes such as atomization or milling, thermal decomposition, and electrochemical processes can be used. Silver powders and palladium powders used in electronic applications are generally manufactured using chemical precipitation processes. In making a silver powder and/or a palladium powder, a metal salt is reduced by using reducing agents such as hydrazine, formaldehyde, hypophosphorous acid, hydroquinone, sodium borohydride, formic acid, and sodium formate. These processes tend to be very hard to control and give irregular shaped particles that are agglomerated.
To obtain the desired silver/palladium ratio, the individual powders are mixed during the manufacture of the thick film paste. In some cases, co-precipitation is used, but the resulting powders are normally just mixtures of silver particles and palladium particles. The present invention uses aerosol decomposition for the production of a silver-palladium alloy.
The aerosol decomposition process involves the conversion of a precursor solution to a powder. The process involves the generation of droplets, transport of the droplets with a gas into a heated reactor, the removal of the solvent by evaporation, the decomposition of the salt to form a porous solid particle, and then the densification of the particle to give fully dense, spherical pure particles. Conditions are such that there is no droplet-to-droplet or particle-to-particle interaction.
The major problem that has limited successful application of the aerosol decomposition process for powder generation is lack of control over particle morphology. In particular, it was the requirement that the material must be treated above its melting point to form fully dense particles and that operating below the melting point tended to give impure, hollow-type particles which were not densified.